1. Field of the Invention
This invention generally relates to clamping circuits in which a recurring wave form is adjusted so as to have its mean or zero value maintained at a preselected level. The invention more specifically applies to systems for digitally clamping binary encoded signals, particularly pulse code modulated video signals, in which the clamping occurs during the horizontal blanking interval of the video signals.
2. Background of the Invention
It has already been proposed in co-pending U.S. application Ser. No. 808,702, filed June 21, 1977, now U.S. Pat. No. 4,124,869, to clamp a pulse code modulated video signal by determining a binary difference value between a predetermined nominal binary value, and the actual binary value of a pulse code modulated signal during the horizontal scanning interval of that signal. The difference value was to be added to the pulse code modulated video signal as a corrective value in the active range of the horizontal period. However, this process has the disadvantage that in those situations in which noise is superimposed over the pulse code modulated video signal, the interference results in alternating clamping level changes which rapidly occur, thereby causing a "lineation" of the clamped image signal.
It is, therefore, an object of the present invention to define a process and apparatus for digitally clamping pulse code modulated video signals which would avoid rapid changes in clamping level and thereby "time-average" the clamping effect on that signal. It is a further object of the present invention to provide various means and methods for digitally time-averaging clamping binary encoded signals, and thereby introducing a time constant which, in its effect, is similar to that found in analog clamping circuits.